Provide the 'serverRoot' in jupyter-config-data for tests

[brichet]

When testing extensions, it can be useful to retrieve the serverRoot from jupyter-config-data script in DOM.
It could be added in page_config with a condition:

notebook/notebook/app.py

Lines 61 to 64 in 2f26f1b

	server_root = self.settings.get("server_root_dir", "")
	server_root = server_root.replace(os.sep, "/")
	server_root = os.path.normpath(os.path.expanduser(server_root))
	try:

A simple solution without adding a new option could be to add it if the option exposeAppInBrowser is True (if this is an option for debugging only).

[brichet]

Closing this issue as not planned.
Discussion in #6604